Electrolyte circulation subsystem

ABSTRACT

An improved electrolyte circulation subsystem which reduces and minimizes the effect of parasitic currents in secondary batteries having a plurality of cells connected electrically in series and a common electrolyte in communication with the cells is described. The improved electrolyte circulation subsystem includes means for pumping an electrolyte, and manifold means for conveying electrolyte to a plurality of cells connected electrically in series. The manifold means generally comprises an outer tube formed with an outlet port at each end thereof, and an inner tube concentrically disposed within the outer tube generally along one-half of the outer tube length. The inner tube is in fluid communication with the pumping means at a first end thereof and is in fluid communication with the outer tube at a second end thereof. The inner tube also has means associated with the second end for generally equally diverting the flow of the electrolyte through the inner tube to each of the outlet ports of the outer tube. The electrolyte circulation subsystem also includes a separate conduit means in fluid communication with each of the outlet ports of the outer tube for individually distributing electrolyte from the outlet tube to generally one-half of the cells.

The present invention relates generally to electrochemical systems, andparticularly to an improved electrolyte circulation subsystem inmultiple-cell secondary batteries having a common electrolyte.

BACKGROUND AND SUMMARY OF THE INVENTION

Electrochemical devices or systems of the type referred to hereininclude one or more of the metal-halogen battery systems, such as azinc-chloride battery system. These metal-halogen battery systemsgenerally are comprised of three basic components, namely an electrodestack section, an electrolyte circulation subsystem, and a storesubsystem. The electrode stack section typically includes a plurality ofcells connected together electrically in various series and parallelcombinations to achieve a desired operating voltage and current at thebattery terminals over a charge/discharge battery cycle. Each cell iscomprised of a positive and negative electrode which are both in contactwith an aqueous metal helaide electrolyte. The electrolyte circulationsubsystem operates to circulate the metalhalide electrolyte from areservoir through each of the cells in the electrode stack in order toreplenish the metal and halogen electrolyte ionic components as they areoxidized or reduced in the cells during the battery cycle. In a closed,self-contained metal-halogen battery system, the storage subsystem isused to contain the halogen gas or liquid which is liberated from thecells during the charging of the battery system for subsequent return tothe cells during the discharging of the battery system. In thezinc-chloride battery system, chlorine gas is liberated from thepositive electrodes of the cells and stored in the form of chlorinehydrate. Chlorine hydrate is a solid which is formed by the storesubsystem in a process analogous to the process of freezing water wherechlorine is included in the ice crystal.

With reference to the general operation of a zinc-chloride batterysystem, an electrolyte pump operates to circulate the aqueouszinc-chloride electrolyte from a reservoir to each of the positive or"chlorine" electrodes in the electrode stack. These chlorine electrodesare typically made of porous graphite, and the electrolyte passesthrough the pores of the chlorine electrodes into a space between thechlorine electrodes and the opposing negative or "zinc" electrodes. Theelectrolyte then flows up between the opposing electrodes or otherwiseout of the cells in the electrode stack and back to the electrolytereservoir or sump.

During the charging of the zinc-chloride battery system, zinc metal isdeposited on the zinc electrode substrates and chlorine gas is liberatedor generated at the chlorine electrode. The chlorine gas is collected ina suitable conduit, and then mixed with a chilled liquid to formchlorine hydrate. A gas pump is typically employed to draw the chlorinegas from the electrode stack and mix it with the chilled liquid, (i.e.,generally either zinc-chloride electrolyte or water). The chlorinehydrate is then deposited in a store container until the battery systemis to be discharged.

During the discharging of the zinc-chloride battery system, the chlorinehydrate is decomposed by permitting the store temperature to increase,such as by circulating a warm liquid through the store container. Thechlorine gas thereby recovered is returned to the electrode stack viathe electrolyte circulation subsystem, were it is reduced at thechlorine electrodes. Simultaneously, the zinc metal is dissolved off ofthe zinc electrode substrates, and power is available at the batteryterminals.

Over the course of the zinc-chloride battery charge/discharge cycle, theconcentration of the electrolyte varies as a result of theelectrochemical reactions occurring at the electrodes in the cells ofthe electrode stack. At the beginning of charge, the concentration ofzinc-chloride in the aqueous electrolyte may typically be 2.0 molar. Asthe charging portion of the cycle progresses, the electrolyteconcentration will gradually decrease with the depletion of zinc andchloride ions from the electrolyte. When the battery system is fullycharged, the electrolyte concentration will typically be reduced to 0.5molar. Then, as the battery system is discharged, the electrolyteconcentration will gradually swing upwardly and return to the original2.0 molar concentration when the battery system is completely or fullydischarged.

Further discussion of the structure and operation of zinc-chloridebattery systems may be found in the following commonly assigned patents:Symons U.S. Pat. No. 3,713,888 entitled "Process For Electrical EnergyUsing Solid Halogen Hydrates"; Symons U.S. Pat. No. 3,809,578 entitled"Process For Forming And Storing Halogen Hydrate In A Battery"; Carr etal U.S. Pat. No. 3,909,298 entitled "Batteries Comprising VentedElectrodes And Method of Using Same"; Carr U.S. Pat. No. 4,100,332entitled "Comb Type Bipolar Electrode Elements And Battery StackThereof". Such systems are also described in published reports preparedby the assignee herein, such as "Development of the Zinc-ChlorideBattery for Utility Applications", Interim Report EM-1417, May 1980, and"Development of the Zinc-Chloride Battery for Utility Applications",Interim Report EM-1051, April 1979, both prepared for the Electric PowerResearch Institute, Palo Alto, Calif. The specific teachings of theaforementioned cited references are incorporated herein by reference.

Additional advantages and features for the present invention will becomeapparent from a reading of the detailed description of the preferredembodiments which make reference to the following set of drawings inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side elevation view of one embodiment of a zinc-chloridebattery system according to the present invention.

FIG. 1B is a front elevation view of the battery system shown in FIG.1A.

FIG. 2 is a schematic diagram of the battery system shown in FIGS. 1Aand 1B.

FIG. 3 is a cutaway perspective view of the stack and electrolytecirculation subsystems for the battery system shown in FIGS. 1 and 2.

FIG. 4 is a schematic diagram of the electrolyte circulation subsystemshown in the FIGS. 2 and 3.

FIG. 5 is a front elevation view of the stack and electrolytecirculation subsystems shown in FIG. 3 with the end cap removed.

FIG. 6 is a fragmentary plan view of the electrolyte distributionmanifold for the electrolyte circulation subsystem.

FIG. 7 is a framentary cross-sectional view of the manifold shown inFIG. 10 taken along lines 7--7.

FIG. 8 is a graph comparing parasitic current values associated with twoelectrolyte circulation schemes.

FIG. 9 is an exploded perspective view of an electrode assembly whichforms the basic building block of the battery stack shown in FIGS. 2 and3.

FIG. 10 is a fragmentary exploded view of an "open" submodule for azinc-chloride battery stack.

FIG. 11 is a perspective view of a comb assembly employed in thesubmodule shown in FIG. 10.

FIG. 12 is a cutaway perspective view of a "closed" submodule for thezinc-chloride battery stack shown in FIGS. 2 and 3.

FIG. 13 is a fragmentary top elevation view of the submodule shown inFIG. 12.

FIG. 14 is a cross-sectional view of the submodule shown in FIG. 13taken along lines 14--14.

FIG. 15 is a cross-sectional view of the submodule shown in FIG. 13taken along lines 15--15.

FIG. 16 is a horizontal cross-sectional view of the submodule shown inFIG. 13 taken along lines 16--16.

FIG. 17 is a cutaway perspective view of a store subsystem employing aconventional decomposition heat exchanger.

FIG. 18 is a schematic diagram of a self draining heat decompositionheat exchanger for a zinc-chloride battery system.

FIG. 19 is a cutaway perspective view of the store subsystem for thebattery system shown in FIGS. 1 and 2.

FIG. 20 is an alternative embodiment of a zinc-chloride battery systemaccording to the present invention.

FIG. 21 is a high density arrangement of a plurality of battery systemsof the type shown in FIG. 20.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1A and 1B, two elevation views of a zinc-chloridebattery system 10 in accordance with the present invention are shown.The various components of the battery system 10 are housed by twointerconnected cylindrical vessels 12 and 14, which may best beillustrated by the schematic diagram of FIG. 2. The upper vessel or case12 is used to contain the chlorine hydrate store subsystem generallydesignated by reference numeral 16. The lower vessel or case 14 is usedto contain both the battery stack 18 and the electrolyte circulationsubsystem generally designated by reference numeral 20.

The cylindrical vessels 12 and 14 are supported by a battery rackstructure 22. The vessels 12 and 14 are preferably made fromfiberglass-reinforced plastic (FRP) with an internal polyvinyl chloride(PVC) liner bonded thereto which is chemically resistant or inert to theelectrolyte and the other chemical entities present within thesevessels. The vessels 12 and 14 are interconnected by four fluid exchangelines or conduits 24, 26, 28 and 30, and the direction of fluid flowthrough these lines are indicated by the appropriate arrows.Additionally, the battery system 10 is provided with four refrigerantlines or conduits 32, 34, 36 and 38. Refrigerant lines 32 and 34 areused to supply a coolant to the store subsystem 16 during the chargingof the battery system for reducing the temperature inside vessel 12 tothe appropriate level to form chlorine hydrate. Refrigerant lines 36 and38 are used to supply a coolant to the sump or electrolyte reservoir 40of the electrolyte circulation subsystem 20 to control the temperatureof the zinc-chloride electrolyte.

Referring now to FIGS. 2 and 3, the electrolyte circulation subsystem 20will now be described. The electrolyte circulation subsystem 20 includesan ovate electrolyte pump "P1" which is mounted to the front end cap 42of the bottom vessel 14 below the electrolyte level in the sump 40. Theelectrolyte pump P1 is driven by an electric motor 44 which ismagnetically coupled to the electrolyte pump. The electrolyte pump 42 ispreferably of the centrifugal type manufactured by Ingersoll-Rand. Theelectrolyte pump 42 is adapted to draw electrolyte from the sump 40through a titanium protective screen filter 46, and discharge theelectrolyte axially through a slip joint 48 into a unique center-feedelectrolyte distribution manifold 50.

The manifold 50 is used to distribute electrolyte to each of the unitcells in the pair of submodules 52 and 54 which comprise the batterystack 18. The manifold 50 not only uniformly distributes the electrolyteto each of the unit cells in the submodule 52 and 54, but also acts tocontrol and suppress the flow of parasitic currents which flow in theelectrolyte circulation subsystem. Parasitic currents are thoseelectrical currents which flow in the conductive paths created by thenetwork of electrolyte connections linking the cells. With the provisionof the manifold 50, especially in combination with inlet and outletcrossed distribution tubes arrangements, significant improvements in thesuppression of parasitic currents have been achieved which will bediscussed more fully below.

Referring to FIG. 4, a schematic diagram of one embodiment of theelectrolyte circulation subsystem 20 is shown, which particularlyillustrates the flow of electrolyte through the manifold 50. Referencemay also be made to FIG. 5 which illustrates a front end view ofelectrolyte circulation subsystem in association with the submodules 52and 54, and FIGS. 6 and 7 which illustrate two views of the manifold 50.The manifold 50 includes a central portion 56 comprising a pair ofconcentric tubes, namely inner tube 58 and outer tube 60. Inner tube 58is in fluid communication with the electrolyte pump P1 at a first end 62thereof and includes an open opposite end 64 which is fitted with anin-line wire mesh electrolyte filter 66. Filter 66 includes a perforatedprotective outer jacket 68 and terminates at its distal end in a cap orplug 70. Electrolyte is pumped through inner tube 58 and passes outthrough filter 66 into outer tube 60. Outer tube 60 is fitted with anend cap 72 which includes a fluid conduit path 74 for conducting fluidaround the inner end cap 70 and thence into conduit 76. Conduit 76 issealed as at end plate 78 and includes a pair of fittings 80 and 82 forbifurcating the electrolyte fluid path. Similarly, outer tube 60includes a pair of fittings 84 and 86 at the end adjacent electrolytepump P1 for providing yet another bifurcated fluid path. Electrolyte isthus pumped out through filter 66 and into outer tube 60, whereupon halfof the fluid is transmitted through tube 76 generally away fromelectrolyte pump P1, while the other half is transmitted through outertube 60 generally toward electrolyte pump P1. A fittings 80-82 and atfittings 84-86 the electrolyte flow is again split into fourdistribution paths for feeding individually both left and right halvesof the two submodules 52 and 54.

With specific reference to FIG. 4, the crossed inlet distribution tubesarrangement may now be explained. It will be seen in FIG. 4 that, forexample, the distribution tube 88, which is connected to fitting 82,feeds the left half 90 of submodule 54, the left half being physicallymore remote from fitting 82 than the right half 92. Similarly, fitting86 is coupled via distribution tube 94 to the right half 92 of submodule54. Hence the electrical circuit path between left and right paths ofsubmodule 54 are quite elongated and provide substantial resistanceagainst parasitic current flow. For example, parasitic current flowbetween unit cell feeder tubes 96 and 98 must travel the entire distancethrough tubes 88, fitting 82, tube 76, outer tube 60, fitting 86, andtube 94 in order to complete the shunt circuit. Although the length ofthis shunt circuit is substantial and the electrical resistance istherefore high, the fluid circuit thus described including the crossedinlet distribution tubes 88 and 94 provides remarkably little burden onelectrolyte pump P1. Hence electrolyte pump P1 can be of a lowerhorsepower with a resultant improvement of the overall efficiency of thesystem.

The outlet portion of the electrolyte circulation subsystem 20 in theembodiment of FIG. 4 also employs the crossed outlet distribution tubesarrangement in order to increase the electrical resistance to parasiticcurrent flow, in a similar manner as described above with respect tocrossed inlet distribution tubes 88 and 94. With reference to FIG. 4 itwill be seen that the left and right halves of each submodule, such asleft and right halves 90 and 92, are cross-coupled to outlet tubes 100and 102, respectively, in a fashion similar to the cross-coupled centerfeed inlet portion. Alternatively, the outlet portion of the electrolyteciruclation subsystem 20 may employ a cascade canopy 104 as illustratedin FIGS. 3 and 5. In this alternative outlet arrangement each individualunit cell discharges through an outlet port 106 via discharge tube 108and thence through orifices 110 onto the upper surface of the cascadecanopy 104. The electrolyte then spills over the canopy 104, like rainwater upon a shingled roof, spreading outwardly as it falls into sump40, which improves the absorption of gaseous chlorine by the electrolyteduring the discharge cycle.

The manifold 50, the inlet crossed distribution tubes arrangement andthe outlet crossed collection tubes arrangement may broadly by viewed asdifferent facets or building blocks of a more general principle orarrangement which may be called the center feed principle orarrangement, which is best explained by reference to prior artelectrolyte distribution practices. In prior art zinc-chloride batterysystems, electrolyte is typically delivered to each unit cell in asubmodule comprised of series-connected unit cells via a common header,such a substantially continuous manifold or distribution tube, havingrelatively low electrolyte resistance from one end of the header to theother. This end feed arrangement allows relatively large parasiticcurrents to develop in virtually every inter-cell shunt circuit in thesubmodule. In contrast, the electrolyte distribution system illustratedin FIG. 4 delivers electrolyte to a single submodule (such as submodule54 for example) composed of series-connected unit cells by splitting theelectrolyte flow in half, and delivering (or removing) each half-flowthrough a physically distinct header (for example tubes 60 and 76 inmanifold 50, or inlet distribution tubes 88 and 94) having a relativelylarge resistance to parasitic current flow. Thus, in comparison to thelarge parasitic currents existing between the commonly fed halves of thesubmodule in the prior art and feed arrangements, all shunt circuits inFIG. 4 existing between the separately fed (with electrolyte) havles ofthe submodule are dramatically reduced. Broadly speaking, then, thecenter feed arrangement may be said to encompass any electrolytedistribution scheme wherein the flow of electrolyte to a singlesubmodule of series-connected unit cells is divided into two (or more)roughly equal portions and thereafter segregated for distribution (orcollection) through two separate, electrically isolated headers havingrelatively large electrical resistances wherein each header supplieselectrolyte to (or collects electrolyte from) a plurality ofseries-connected unit cells, and the parasitic currents caused byinter-cell shunt circuits between the separately fed portions of thesubmodule are substantially reduced relative to an end feed electrolytedistribution arrangement. The electrolyte circulation subsystem 20 ofthe battery system 10 disclosed herein was specifically designed so asto incorporate the foregoing center feed principle and to maximize theadvantageous reduction in parasitic currents obtainable by utilizing acenter feed arrangement.

The effectiveness of the above-described center feed electrolytedistribution arrangement is exemplified by reference to FIG. 8. FIG. 8is a graphical representation of typical shunt or parasitic currentvalues displayed along the ordinate as a function of the cell numberdisplayed along the abscissa. For illustration purposes a battery havingthirty unit cells connected electrically in series has been assumed,although it should be understood that the same advantageous results areobtainable in batteries having a different number of unit cellsconnected electrically in series. As related above, all of the cells inthe battery are served by one electrolyte pump through a common supplyand return manifolding. This common electrolyte manifolding provides anelectrically conductive path through which current will pass when avoltage is present across the battery terminals and electrolytecirculation subsystem 20 including the battery stack is full ofelectrolyte. This shunt current reduces the effective current flowingthrough the cells during charge and causes cells in the battery to selfdischarge during discharge at different rates. In general, this resultsin faster depletion of zinc from the electrodes of cells in the centerof the battery stack, and can cause measurable differences in thecoulombic efficiency of cells within the battery stack.

Knowing the resistivity of the electrolyte and the sizes of differentportions of the electrolyte circulation subsystem 20, the effectiveelectrical resistances of the various sections can be calculated. Anequivalent electrical circuit model may then be constructed, if desired,in accordance with the teachings of U.S. Pat. No. 4,371,825, issued onFeb. 1, 1983 to Chi et al, entitled "Method Of Minimizing The Effects OfParasitic Currents", which is hereby incorporated by reference. FIG. 8is a graph comparing parasitic current values during charging which werecalculated from such an electrical circuit model. FIG. 8 includes acurve 112 which represents the parasitic current distribution for abattery system having a prior art end-feed electrolyte distributionmanifold, and a curve 114 which represents the parasitic currentdistribution for a battery system in accordance with the presentinvention having a center-feed electrolyte distribution manifold. It isimportant to note that the total parasitic current flow of curve 112 isnot only greater than that for curve 114, but curve 114 indicates thatthe parasitic current distribution is considerably more uniform when thecenter-feed manifold is utilized. This benefit of the center-feedmanifold is advantageous because it is not only desirable to minimizeparasitic current flow, but is is also desirable to have a uniformdistribution of the parasitic currents across the battery stack in orderto achieve a substantially uniform coulombic efficiency for each of theunit cells in the battery stack.

Referring to FIG. 9, an exploded view of a zinc-chloride batteryelectrode assembly 200 is shown which forms the basic building block ofthe battery stack 18. Electrode assembly 200 generally comprises a pairof porous graphite positive or chlorine electrodes 202 and 204, a densegraphite negative or zinc electrode 206, and plasitc frame members 208and 210. The positive electrodes 202 and 204 are adapted to slide intochannels 212 and 214, respectively, in the frame member 208 such thatthe frame member supports these two electrodes along the top and bottomedges as well as along one of the side edges. The frame member 208operates to align the positive electrodes 202 and 204 in parallel andprovides an internal cavity between these electrodes. The frame member208 is also formed to nestingly receive the frame member 210 between thepositive electrodes 202 and 204.

The frame member 210 includes a plastic-feed tube 216 for conveyingelectrolyte from a unit cell manifold to the internal cavity between thepositive electrodes 202 and 204. The frame member 208 is also formedwith a channel 218 which is adapted to receive a side edge of thenegative electrode 206 and align the negative electrode 206 in parallelwith the positive electrode 202. Accordingly, it will be appreciatedthat the frame member 208 serves to align and separate the positiveelectrodes 202 and 204 from each other, and also to align and separatethe negative electrode 206 from the positive electrode 202. Theseparation between the negative electrode 206 and the positive electrode202 is referred to as the inter-electrode gap which may generally rangefrom about 40 mils (1 mm) to about 250 mils (6 mm) and is preferablyabout 129 mils (3.3 mm).

The frame member 208 also serves to control the edge effects of thepositive electrodes 202 and 204 by providing an integral maksing orscreening around the edges of the positive electrodes in order to modifythe electrochemical activity along these edges. Generally speaking, thechannels 212, 214 and 218 are formed such that the apparent surface areaof the positive electrodes is smaller in comparison with the apparentsurface area of the negative electrodes. A more detailed discussion ofmasking edge effects may be found in the commonly assigned Carr et al.U.S. Pat. No. 4,241,150, entitled "Method for Control of Edge Effects ofOxidant Electrode", which is hereby incorporated by reference.

It should also be noted that the frame member 208 includes an orifice220 at the top thereof for venting any undissolved chlorine gas whichcould otherwise be trapped in the internal cavity between the positiveelectrodes 202 and 204. Additionally, the frame member 208 is formedwith a pair of opposing, vertically extending spacing ribs 222 and 224.The ribs 222 and 224 restrain any tendency of the positive electrodes202 and 204 to bow outwardly, and insure that the desiredinter-electrode gap between the positive and negative electrodes ismaintained. The integrity of this inter-electrode gap is importantbecause it has been found that with increased gaps on the order of 129mils the electrical current density for the battery system may besignificantly increased. Also such increased gaps permit higher zincloadings on the negative electrodes, which in turn means thatsubstantial cost savings can be achieved through the reduction in thenumber of electrodes required to store an equivalent amount ofelectrical energy.

The feed tube 216 of the electrode assembly 200 is press fit into asocket which is formed into an upwardly extending nipple portion 226 ofthe frame member 210. Additionally, the bottom end of the feed tube 216is trapped between an upwardly extending clip portion 228 and thesupport channel portion 230 of the frame member 210. It should also benoted that the bottom end of the support channel portion 230 of theframe member 210 is shaped to mate with the bottom end of the internalseparator portion 232 of the frame member 208. This contoured shaping atthe bottom end operates in combination with a generally horizontallyextending flange portion 234 of the frame member 210 at the top thereofto lock the frame member 210 to the frame member 208.

With respect to the materials which may be used to construct theelectrode assembly 200, it is preferred that the positive electrodes 202and 204 be constructed from Union Carbide Corp. PG-60 or TS-1697graphite, or Airco Carbon Co. S-1029 or S-1517 graphite. With respect tothe negative electrode 206, it is preferred that this electrode beconstructed from Union Carbide Corp. ECL grade graphite or alternativegrades such as ATR or ATJ graphite herein. With respect to the framemembers 208 and 210 and the tube 216, these components (as well as theother plastic components to be described below) may be constructed fromany suitable electrically nonconductive material which is chemicallyresistant or inert to the electrolyte and other chemical entities withwhich they will come into contact. While it is preferred that the framemembers 208 and 210 be constructed from General Tire and Rubber Corp.Boltaron® polyvinylchloride or B. F. Goodrich Corp. Geon®Polyvinyl-chloride and the tube 216 from DuPont Teflon®)polytetrafluoro-ethylene), other suitable plastic materials may beemployed such as Penwalt Kynar® (polyvinylidene fluoride) or any of theother appropriate materials described in Section 33 of The Developmentof the Zinc Chloride Battery For Utility Applications, April 1979 reportidentified earlier.

Referring to FIG. 10, an exploded view of an "open" submodule 236 for azinc-chloride battery stack is shown. The submodule 236 generallycomprises a zinc termination comb assembly 238, a chlorine terminationcomb assembly 240, and one or more bipolar intermediate comb assemblies242. While the submodule 236 is shown with only one intermediate combassembly 242, it should be appreciated that the submodule may beexpanded by merely providing for more intermediate comb assemblies. Asshown in FIG. 10, the submodule 236 includes two "unit" cells connectedelectrically in series. Each of these unit cells comprise a number ofsingle cells (i.e., a positive electrode and an opposing negativeelectrode) connected electrically in parallel.

The intermediate comb assembly 242, which may best be seen withreference to FIG. 11, includes an electrically conductive bus member 244(i.e. constructed from dense graphite) which has two generally planaropposing faces and a plastic frame 246 generally disposed around theedges of the bus member to provide an ionic seal between adjacent unitcells. Frame 246 is preferably formed by injection molding PVC about theedges of bus member 244. A pair of opposed longitudinally extendinggrooves 247 may be used to provide a mechanical interlock between thisPVC encapsulation and the edges of bus member 244. A plurality ofpositive electrode structures 248 are attached via a press orinterference fit connection to one exterior face of the bus member 244,which is provided with spaced vertical grooves 249, while a plurality ofnegative electrodes 250 are attached to the other face of the bus memberin a similar fashion. Each of the positive electrode structures 248 areconstructed in accordance with the electrode assembly 200 of FIG. 9, andinclude the positive electrodes 202 and 204, and the plastic framemembers 208 and 210. A unit cell electrolyte distribution manifold 252is ultrasonically welded or otherwise secured to the top section of eachframe 246 such that electrolyte may be conveyed to the feed tubes 216.Specifically, the nipples 226 extending from the top of the framemembers 210 are inserted through holes in the bottom tray 254 of themanifold 252. These nipples 226 are then welded by thermal swaging tothe bottom tray 254 of the manifold 252 to provide a leak-proofconnection.

In order that each unit cell may be separately sealed, a plastic tray256 as shown in FIG. 10 is welded or otherwise secured to the bus barframe 246 in a fluid tight connection. A return path for the electrolytesupplied to each of the unit cells is provided by a collection cup 258and a discharge serpentine channel plate 260 which are adapted toreceive the electrolyte flowing from the unit cell and direct thiselectrolyte to the reservoir or sump. As in the case of the otherplastic frame members or components, the collection cup 258 and thedischarge serpentine channel plate 260 are welded or otherwise secured(such as by solvent bonding) to the tray 256 in a fluid tightconnection.

As illustrated in FIG. 11, the unit cell distribution manifold 252 alsoincludes a top cover 262 which is secured to the bottom tray 254 bywelding or solvent bonding. An important feature of the manifold 252 isthe provision of a plastic perforated screen 264 which extends along thecomplete length of the manifold between the bottom tray 254 and the topcover 262. The perforations in the screen 264 are selected to besuitably smaller than the diameter of the opening in the nipples 226 ofthe frame member 210, so that any particles which could plug or obstructfluid flow through the feed tubes 216 will be filtered by the screen264. The screen 264 is preferably constructed from Kynar® and ispreferably bent over in a generally U-shape. It should also noted thatthe manifold 252 is also be provided with a suitable orifice 265 (shownin FIG. 12) for permitting any gas which could otherwise be trapped inthe manifold to escape. The location of orifices 265 near the outsideedges of the unit cell also assures that sufficient electrolyte flowwill occur adjacent the outermost electrodes of the unit cell.

In FIG. 10, the aforementioned plastic components 252 through 264 areshown in an assembled state with reference to the chlorine terminationcomb assembly 240. The chlorine termination comb assembly 240 is similarin construction to the intermediate comb assembly 242 except that thechlorine termination comb assembly is not provided with a plurality ofnegative electrodes 250 along one face of the bus bar 244. However, thechlorine termination comb assembly 240 includes a plurality ofelectrical terminals mounted to the bus bar 244 to facilitate externalelectrical connections to the submodule 236. These electrical terminals,generally designated by reference numeral 266, are illustrated withreference to the zinc termination comb assembly 238. The zinctermination comb assembly 238 simply comprises a bus bar whose edges andexternal face are enclosed in a plastic frame and a plurality ofnegative electrodes attached on the internal face thereof. In anassembled state, the positive electrode structures 248 of theintermediate comb assembly 242 will be interdigitated with the negativeelectrodes 250 of the zinc termination comb assembly 238, and thenegative electrodes 250 of the intermediate comb assembly 242 will beinterdigitated with the positive electrode structures 248 of thechloride termination comb assembly 240. Accordingly, the positiveelectrode structures 248 of the intermediate comb assembly 242 and thenegative electrodes 250 of the zinc termination comb assembly 238 willform one unit cell, and the negative electrodes 250 of the intermediatecomb assembly 242 and the positive electrode structures 248 of thechlorine termination comb assembly 240 will form the other unit cell ofthe submodule 236.

Referring to FIG. 12, a cutaway perspective view of the "closed"submodule 54 for the battery stack 18 of FIGS. 2 and 3 is shown. Theconstruction of the submodule 54 is similar to the submodule 236 of FIG.10 in several respects. The principal difference between these twosubmodules is that the submodule 236 is generally open at the topthereof to allow chlorine gas (as well as any other gases) to beliberated from the unit cells; whereas, the submodule 54 is generallyclosed at the top thereof to control the flow of fluid from unit cells.The submodule 54 is comprised of twenty-four cells connectedelectrically in series. These unit cells are generally designated byreference 300.

Referring additionally to FIGS. 13 through 16, several views of the zinctermination unit cell 300 for the submodule 54 are shown, whichparticularly illustrate the plastic top section 310 thereof. The topsection is welded or otherwise sealably secured to a three sided traysection 311 to form a substantially closed compartment for the unitcell. The top section 310 includes a supply port 312 which is connectedto electrolyte distribution tube 88 via a unit cell feed tube 313. Asimilar electrolyte connection may best be seen with reference to FIG.3, which shows the supply port 314 of a unit cell 316 of the submodule52 connected to the electrolyte distribution tube 318 via a feed tube320.

The top section 310 of the unit cell 300 also includes an outlet port322 which is connected to the cascade canopy 324 via an outlet tube 326.As may best be seen with respect to FIG. 14, the top section furtherincludes a generally horizontally extending top wall 328 which isintegrally formed with a downwardly extending serpentine partitionportion 330. The serpentine partition portion 330 is used to form aserpentine-channel discharge manifold 332 in combination with a bottomcover plate 334 which is secured thereto in a generally fluid tightseal. The opening 336 of the discharge manifold permits chlorine gas andelectrolyte to flow out of the unit cell 300 as may best be seen withreference to FIG. 16.

The top section 310 of the unit cell 300 additionally includes a unitcell feed manifold 338, which is generally comprised of a top cover 340and a bottom tray 342 secured thereto in fluid tight relationship. Thetop cover 340 includes an upper cylindrical portion 344 which is adaptedto extend through an orifice in the top wall 328 of the top section 310.The supply port 312 is adapted to slide over and be secured to thecylindrical portion 344. The top cover is also formed with elongated,downwardly extending partition portions 346 and 348 which direct theflow of electrolyte through the manifold 338 in cooperation with thebottom tray 342. Interposed between the top cover 340 and the bottomtray is a screen 350 for filtering the flow of electrolyte to the unitcell 300. The bottom tray 342 is formed with a plurality of holes 352through which the nipples 226 of electrode frames 208 extend in order tobe welded to the bottom tray and permit electrolyte flow to the internalcavities between the chlorine or positive electrodes 202 and 204.

Referring collectively to FIGS. 3, 4, and 9-16, the uniformity ofelectrolyte distribution amongst all of the individual chlorineelectrode pairs contained in electrode assemblies 200 of the batterysystem 10 may now be explained. As shown in the Figures just mentioned,the electrolyte circulation subsystem 20 of the battery system 10 iscomprised of myriad large and small manifolds, serpentines and varioussize distribution tubes, all of which have been sized to present ratherlow hydraulic resistance to the amount of electrolyte designed to flowtherethrough in comparison to relatively high hydraulic resistance toflow presented by each feed tube 216 (see FIG. 9) in the battery stack18. On account of the foregoing design, there exists substantially equalhydraulic pressure in all unit cell manifolds 252 (see FIG. 11) and inall serpentine-channel discharge manifolds of each submodule (see FIGS.12 and 14), if not both submodules 52 and 54. Accordingly, since thedifferential electrolyte pressure across each feed tube 216 issubstantially the same, and since all feed tubes 216 in the batterystack 18 are of the same length and inner diameter, all electrodeassemblies 200 in each submodule experience substantially equal flowrates.

Similarly, since the flow capacities of all manifolds and distributiontubes in electrolyte circulation subsystem 20 are relatively large incomparison to the flow rates they experience, the differential hydraulicpressure across any given feed tube 216, and therefore the electrolyteflow rate for the electrode assembly 200 it supplies, remainssubstantially uniform over time while the battery system 10 is chargingor discharging.

Referring specifically now to FIG. 15 the unit cell 300 is also shown toinclude a gas relief valve 354 which is secured to the top wall 328 in afluid tight relationship. The relief valve 354 is used to selectivelyvent gas from the interior compartment of the unit cell 300 in responseto the electrolyte level in the unit cell. In particular, the reliefvalve 354 is advantageously used to vent any hydrogen gas which may bepresent in the unit cell compartment when the battery system 10 is in acharge standby mode.

The relief valve 354 is generally comprised of a conical-shaped housing356 having a roughly cone-shaped hollow interior 357, and a buoyantfloat member 358. The housing 356 is formed with an orifice 360 at itstop end for venting gas, and the relief valve 354 is formed at itsbottom end with a pair of tang members 362 and 364 (which may best beseen in FIG. 16) for mechanically locking the relief valve to the topwall 328 in a snap-fit connection. The float member 358 is shaped togenerally conform to the interior surface 366 of the housing 356, sothat the float member will block the flow of fluid from the unit cellcompartment when the float member is moved upwardly into sealingengagement with the housing by the pressure exerted on the float memberby the electrolyte. The float member 358 is also formed with an upwardlyextending stem portion 368 for guiding the upward movement of the floatmember into sealing engagement with the housing 356. Since the floatmember 358 preferably has a hollow interior, a bottom plate 370 isbonded to the cylindrical portion of the float member in order to trap aquantity of air therein. It should also be noted that while the top wall328 of the unit cell compartment is provided with an orifice 372 forcommunicating fluid to the relief valve 354, the orifice is madesuitably smaller than the diameter of the float member 358 in order toprevent the float member from dropping into the discharge manifold 332when the electrolyte level is low. Nevertheless, the orifice 372 mustalso be suitably shaped so as to permit venting even when the floatmember 358 has dropped to the point where it is resting upon the topwall 328.

When the electrolyte is being circulated through the battery system 10,such as during the charging or discharging of the battery system, thedischarge manifolds 332 for each of the unit cells 300 will becomefilled with electrolyte and cause the float members 358 to move upwardlyto the point were the float members 358 seal the orifices 360. Then,when the battery system is switched to a standby mode, for example atthe end of charge or discharge, the electrolyte pump P1 will be turnedoff and electrolyte circulation will cease. This will cause theelectrolyte level in the discharge manifolds 332 to drop to a pointsufficient to re-open the orifices 360 by the downward movement of thefloat members 358. The re-opening at the orifices 360 will permit anygas present in the discharge manifolds 332 or in the gas space betweenthe plates 334 and the tops of the electrode frame members 208 to bevented from the unit cell compartments through the relief valves 354.This automatic venting provision is especially important when thebattery system 10 is placed in a standby mode after the battery systemhas been charged, as it will permit any hydrogen gas evolved at the zincor negative electrodes 206 during this time to be vented from the unitcell compartments. It should also be noted that the relief valve 354 isdesigned, through an appropriate choice of size and density for thefloat member 358, so as to not permit capillary attraction or surfacetension of the electrolyte to hold the bouyant float member 358 up insealing engagement with the housing 356 after electrolyte circulationhas ceased.

Referring to FIGS. 3 and 5, these Figures also illustrate a plastic sled400 which is used to support the submodules 52 and 54 in the lowervessel 14. After the submodules 52 and 54 have been fully assembled withthe various electrolyte distribution and collection components describedabove connected thereto, the sled 400 is then slid into the vessel 14along an elongated rail 402.

FIG. 3 also illustrates the electrical connections which are made to thesubmodules 52 and 54. A set of four power terminals 404 are providedsuch that one power terminal is connected to each end of the submodules52 and 54. Each of these power terminals comprise a titanium clad copperrod 406 which is friction welded to a titanium bar 408. The titaniumbars 408 are attached to the plurality of terminal posts 410 provided ateach end of the submodules 52 and 54. Once attached to the submodules 52and 54, the power terminals 404 are then preferably encased in a plastic(liquid potting resin) envelope that extends outside of the vessel 14.The free ends of the power terminals 404 may then be connected to asuitable D.C. power source for charging the battery system 10 or asuitable load for discharging the battery system.

FIG. 3 also illustrates a glass tube 412 which is used to house asuitable ultraviolet light source, shown in phantom at reference numeral414. The glass tube 412 is adapted to extend outside of the vessel 14 tofacilitate replacement of the ultraviolet light source 414. Theultraviolet light source is adapted to react any hydrogen gas which maybe present in the gas space in the vessel 14 with chlorine gas to formhydrogen chloride.

Referring again to FIG. 2, the interaction between the store subsystem16 of the vessel 12 and the battery stack 18 and electrolyte circulationsubsystem 20 of vessel 14 will now be briefly described. When thebattery system 10 is in a charge mode, the battery stack 18 willgenerate a continuous supply of chorine gas. The chlorine gas will bedrawn from the vessel 14 to the vessel 12 by the gas/hydrate pump "P2"via conduit 26. The pump P2 will then mix the chlorine gas with achilled liquid (preferably water) in the vessel 12 to form chlorinehydrate. When the battery is in a discharge mode, valve "V1" will beopened to permit warm electrolyte from the sump 40 to be pumped throughthe hydrate decomposition heat exchanger "HX2" located in vessel 12 viaconduits 28 and 30. This will cause the hydrate to gradually decomposeand liberate a continuous supply of chlorine gas. When the valve "V2" isopened, the chlorine gas being liberated in the vessel 12 will then betransmitted back to the vessel 14 via conduit 24. This supply ofchlorine gas is then injected into the electrolyte circulation subsystem20 where the gas is dissolved in the electrolyte which is beingdistributed to the battery stack 18. At the end of discharge, all of thechlorine hydrate will have been decomposed and the chlorine gas returnedand consumed in the battery stack 18.

FIG. 17 is a cutaway perspective view showing the equipment arrangementinside the store subsystem 16 depicted in FIGS. 1 and 2. Store 16 iscontained within a short cylindrical case or vessel 12 and is preferablymounted above the stack vessel 14 as shown in FIG. 1. Vessel 12 has anintegral domed end 600 and is closed with a domed cover 602 at the otherend which is bolted to flange 604. In operation, vessel 12 is filledalmost entirely with liquid (preferably water), leaving a relativelysmall gas space 605 best shown by liquid level line 606 in FIG. 2.

Components located within or on the store vessel 12 include gas/hydratepump P2, filter package F, pressure control orifice 622, hydrate formerheat exchanger HX1, and decomposition heat exchanger HX2. Variousfeatures and operating characteristics of these components may now bedescribed.

Gas/hydrate pump P2 with its electric driving motor 608 attached ismounted on boss 610 of domed cover 602. Pump P2 is of "plug-in" styleconstruction with the motor armature magnetically-coupled to the pumpshaft through a plastic pump cover 612, so that no motor or pump shaftprojects through the wall of store vessel 12. The pump P2 is an externalor spur gear type pump manufactured by Ingersoll-Rand and built withplastic gears and housings and graphite bushings.

Pump P2 discharges through conical nozzle 614 directly into the gasspace 605 at the top of vessel 12. Suction portion 616 of pump P2 plugsor couples directly into inlet fitting or coupling 618 rigidly mountedwithin the vessel 12. Gas from battery stack 18 provided via conduit 26and liquid from the store 16 are fed into pump P2 through coupling 618.Chilled liquid (preferably water) is drawn through the coiledtube-in-tube heat exchanger HX1 from the liquid or water reservoir 620of vessel 12.

In order that hydrate crystals, also slurried in water reservoir 620,not be drawn through heat exchanger HX1 and pump P2, a separationleaf-type filter package "F" is employed. Filter F is configured as adouble-walled cylinder and submerged in water reservoir 620. Filter F isconstructed of a heavy-gauge PVC mesh on a rigid ring-like plasticframe, and covered with a sleeve of Teflon-felted cloth, whicheffectively prevents any hydrate crystals from entering the spacebetween the cylinder walls of filter F.

Water enters heat exchanger HX1 from the space between the cylinderwalls of filter F through an orifice 622 sized to allow the desiredliquid flow rate and maintain the internal pressure within heatexchanger HX1 at approximately suction pressure of pump P2, which ispreferably about 11 psia.

Heat exchanger HX1, as shown in a number of the accompanying drawings,is a simple tube-in-tube assembly, which preferably consists of twoconcentric titanium tubes rolled to form a coil 624 as shown in FIG. 17.High-flux coating, commercially available from Union CarbideCorporation, is preferably deposited on the outer surface of the innertube to drastically reduce the superheat required for refrigerantboiling by promoting nucleate boiling, thus effectively increasing theheat transfer coefficient from two to ten-fold. Use of such a coatingallows heat exchanger HX1 to be made more compact than otherwise wouldbe possible.

Refrigerant used in heat exchanger HX1 is preferably Freon 12, and isprovided through refrigerant supply and return lines 32 and 34, whichare shown passing through the domed end 600 of store vessel 12 by way ofpressure-tight sealed bushings. As shown in FIGS. 2 and 17, refrigerantflows through the annulus portion of coil 624 of heat exchanger HX1,while the store liquid flows through the inner tube of coil 624.

The equipment packaged inside store vessel 12 is preferably mounted on aself-locating support frame or sled (not shown) contoured to rest uponthe curved inner surface of the vessel. Such a support frame allows heatexchangers HX1 and HX2, package filter F, pressure control orifice 622,and the pump inlet coupling 618 to be erected outside of store vessel 12so that they may be slipped inside as a complete assembly. The equipmentpackaged inside the store vessel 12 can then be held stationary withrespect to store vessel 12 by various attachments of the equipment tothe store vessel 12 such as the bushings for the two refrigerant lines32 and 34 and the two bushings for lines 28 and 30 going to heatexchanger HX2 (see FIGS. 2 and 19). By utilizing such a support frameand system of attachment points, no assembly work need be accomplishedinside of the store vessel 12.

The various configurations of decomposition heat exchangers HX2 shown inFIGS. l7, 19 and 20 are formed by bending a length of tubing (preferably1/2 or 5/8 inch O.D. titanium tubing) into the desired shape or pattern.For reasons which will be shortly explained in detail, the coil patternselected for the heat exchanger HX2 should allow electrolyte to drainfrom the heat exchanger HX2 when not in use. FIGS. 19 and 20 show twopreferred coil patterns for heat exchanger HX2 designed to ensure suchproper drainage. Experience has shown, for example, that even thegenerally horizontal coil pattern for heat exchanger HX2 shown in FIG.17 does not effectively provide complete drainage of electrolyte fromheat exchanger HX2.

As mentioned earlier, the store vessel 12 itself is preferably made fromFRP with a PVC liner bonded thereto. Since the temperature within storevessel 12 is preferably maintained at approximately ten degrees C.,which may be below ambient temperatures typically encountered in indoorinstallations of the battery system, thermal insulation is preferablyplaced about much of the external surfaces of the store vessel toimprove the overall system energy efficiency. In a preferred embodiment,a one and one-half inch layer of urethane foam designated by the numeral628 covers approximately eighty percent of the exterior of vessel 12,and the foam in turn may be covered by a thin one-eighth inch of FRPlay-up to protect it from damage.

In the battery system 10 of the present invention, a preferredelectrolyte is a two molar concentration of zinc-chloride (measured whenthe battery system 10 is fully discharged), having supporting (i.e.,conductivity-improving) salts of about a four molar concentration ofpotassium chloride and about a one molar concentration of sodiumchloride to increase overall battery system efficiency.

During the normal operation of the battery system 10, the electrolytetemperature in stack vessel 14 preferably maintained between aboutthirty and forty degrees C. Warm electrolyte from sump 40 continuouslycirculating through heat exchanger HX2 typically is not cooled duringits passage through heat exchanger HX2 more than ten degrees C., andthus, precipitation of supporting salts in the electrolyte does notnormally occur at the time. However, whenever heat exchanger HX2 isturned off long enough for the electrolyte within the heat exchanger tocool to near the internal temperature maintained within the store,precipitation of supporting salts and the resultant clogging ofdecomposition heat exchanger HX2 would be a major problem if the highlysalted electrolyte were allowed to remain in this heat exchanger as theless highly salted electrolytes were allowed to do in earlierzinc-chloride battery systems.

To eliminate such problems, the zinc-chloride battery systems shown inthe prior art have been redesigned so that the decomposition heatexchanger HX2 is now self-draining during those periods of time when noflow of electrolyte is required therethrough. To do this without addingany appreciable additional cost, complexity, or control equipment (suchas a control valve and/or pump) to the battery system, the batterysystem 10 is now designed so that heat exchanger HX2 of the storesubsystem 16 is located higher than the sump 40 associated with thebattery stack 18, so that electrolyte will drain from heat exchanger HX2back to the sump 40 when electrolyte flow therethrough is not required.This is preferably accomplished by placing store vessel 12 completelyabove stack vessel 14 as shown in the latest battery system designs inFIGS. 1, 2 and 20.

FIG. 18 is a schematic diagram of the self-draining heat exchangerconcept with the store 630 elevated above the level of electrolyte inthe sump 40. As can be seen by referring to FIG. 18, electrolyte pump P1provides electrolyte to the stack 632. During the discharge mode of thezinc-chloride battery cycle, pump P1 also provides electrolyte to heatexchanger HX2 through conduit 634 by opening decomposition control valveV1, which is normally closed during all other times of the batterycycle. The rate at which heat is provided to the liquid in store 630 byheat exchanger HX2 determines the rate at which chlorine is liberated bythe decomposition of chlorine hydrate in the store. Control valve V1 maybe intermittently opened and closed during the discharge mode tomodulate this heat transfer rate. When the flow of electrolyte isblocked by control valve V1, electrolyte in heat exchanger HX2 drainsinto sump 40 through conduit 636. Those skilled in the art willappreciate that if heat exchanger HX2 is higher than sump 40 and isprovided with sufficient slope and its tubing is of sufficient innerdiameter, electrolyte will naturally drain therefrom, especially sincethe aqueous electrolyte used in zinc-chloride battery systems has aconsistency very much like plain water. However, to promote much fasterdrainage of the relatively small diameter tubing normally used in heatexchanger HX2, a vent means 638 has been added between the outlet ofpump P1 and the inlet 640 of heat exchabger HX2 to allow gas to enterheat exchanger HX2 to replace electrolyte as it drains therefrom. Thegas is preferably drawn, as shown in FIG. 18, from the gas space 642 ofstack vessel 14. In battery systems employing a decomposition controlvalve V1 located basically as shown in FIG. 18, vent 638 must be placeddownstream from the control valve V1.

Vent means 638 in the preferred enbodiment of the zinc-chloride batterysystem of the present invention is a 1/16 inch diameter hole in conduit634, and is connected to gas space 642 in stack vessel 14. Those skilledin the art will appreciate that a larger or smaller size hole could beused for the vent 638. While a 1/32 inch hole could be used for example,a 1/16 inch hole is deemed preferable since it is deemed lesssusceptible to clogging by any small particulate or foreign matter whichmight possibly be present in the electrolyte. An advantage of a smallhole, such as a 1/16 inch diameter hole, over a considerably large holesuch as a 5/16 inch diameter hole, is that its liquid volumetriccapacity is insignificant in comparison to the flow of electrolytethrough the heat exchanger, so that any electrolyte flow through vent638 back to the sump 40 represents a negligible energy loss to thebattery system. Yet, the gaseous volumetric capacity of the vent 638 forsuch a small hole is sufficiently large to ensure fairly rapid drainageof the electrolyte from heat exchanger HX2 before the electrolytetherein cools sufficiently to allow any significant precipitation ofconductivity-improving salts.

FIGS. 18, 19 and 20 show that a heat exchanger HX2 of the self-drainingtype is preferably constructed of three parts: an inlet portion 640, anoutlet portion 644, and a generally helical central portion 646 disposedbetween the inlet and outlet portions. Central portion 646 preferablyslopes substantially continuously downward from the inlet portion 640 tothe outlet portion 644 in order to prevent any electrolyte fromremaining in the central portion of heat exchanger HX2 when theelectrolyte is to be drained therefrom. The angle of the slope may bevaried so long as it is sufficient both to prevent electrolyte fromremaining within heat exchanger HX2, and to allow relatively quickdrainage of the electrolyte before it cools sufficiently to allow anysignificant precipitation. The optimal slope and configuration of heatexchanger HX2 is dependent upon the room available therefor in storevessel 12, the size and length of the tubing used therefor, the lengthof the conduit inter connections between the heat exchanger and the sump40 and electrolyte pump P1, and the size of the hole or orifice for thevent 638. Variations in all of these design details are within thecontemplated scope of the invention.

Vent means 638, rather than being continuously open to the gas space asis shown in FIG. 18, could alternatively be opened and closed as neededthrough the use of a control valve. A simple hole in conduit 634 isdeemed preferable to using control valve approach to venting since sucha vent hole is less costly, simpler, and inherently automatic inoperation.

As can be seen in FIG. 18, some electrolyte may remain in conduit 634between decomposition control valve V1 and vent 638. Because conduit 634is outside of store vessel 12, and is therefore subject to much higherambient temperatures, precipitation of salt therein is not a problem.

Another benefit of the basic self-draining heat exchanger arrangementshown in FIG. 18 is that it takes full advantage of the natural momentumof the electrolyte flowing through the heat exchanger HX2 to helppromote rapid draining of heat exchanger HX2.

FIGS. 1 and 2 show a preferred embodiment of vent 638 and pipingtherefor. Specifically, decomposition control valve V1 and vent 638,which is shown schematically in FIG. 2 as orifice 648, are locatedexterior to both store and stack vessels 12 and 14. The exteriorlocation of conduit 24, 26, 28 and 30, valves V1 and V2, and vent 638,as well as other equipment shown in FIGS. 1 and 2, facilitatetrouble-shooting, maintenance and repair of these items. FIGS. 19 and 20each illustrate a preferred embodiment of an overall lay-out and coilpattern for a self-draining heat exchanger HX2. Tests of self-drainingheat exchanger HX2 arrangement described above with respect to FIG. 18have shown it to be very effective in preventing precipitation of saltsin heat exchanger HX2 and the clogging problem resulting therefrom.

FIG. 20 shows an alternative embodiment of the store subsystem that isbeing designed for large commercial installations such as electricalutility load-leveling applications. The principles of operation andconstruction techniques of the battery system shown in FIG. 20 arebasically the same as those shown for the battery system of FIG. 1. Theupright position of store vessel 12 in FIG. 20, in conjunction with thereduction of the diameter of the store vessel to match the diameter ofthe stack vessel 14, provides a considerably more compact stackingarrangement for multiple battery systems used in large applications likea commercial load-leveling battery plant. One such compact stackingarrangement, which beneficially provides a rather high battery systemdensity per unit volume, is shown in FIG. 21. To provide for anincreased energy capacity, each individual battery system shown in FIG.20 has its battery stack 18 within the stack vessel 14 increased from 72inches (as shown in the battery stack of FIG. 2) to 92 inches.Similarly, other components such as the three heat exchangers HX1, HX2and HX3 are increased in size to accomodate the increased energycapacity.

While it will be appreciated that the preferred embodiments of theinvention disclosed are well calculated to fulfill the objects abovestated, it will be appreciated that the invention is susceptible tomodification, variation and change without departing from the properscope or fair meaning of the subjoined claims.

What is claimed is:
 1. In an electrochemical system having a pluralityof cells connected electrically in series, an electrolyte circulationsubsystem, comprising:means for pumping an electrolyte; manifold meansfor conveying electrolyte to said cells, said manifold means having anouter tube formed with an outlet port at each end thereof, and an innertube concentrically disposed within said outer tube generally alongone-half of the length of said outer tube, said inner tube being influid communication with said pumping means at a first end thereof andin fluid communication with said outer tube at a second end thereof,said inner tube also having means associated with said second end forgenerally equally diverting the flow of said electrolyte through saidinner tube to each of said outlet ports of said outer tube; separateconduit means in fluid communication with each of said outlet ports ofsaid outer tube for individually distributing electrolyte from saidoutlet tube to generally one-half of said cells.
 2. A manifold forconveying electrolyte to an electrochemical system having first andsecond cells electrically connected in series, comprising:first headerfor delivering electrolyte to said first cell and second header fordelivering electrolyte to said second cell, wherein said first andsecond headers define physically separate, electrically isolated pathsfor minimizing current flow directly between said first and secondheaders.
 3. The manifold of claim 2 wherein said first and secondheaders define paths of substantially equal length.
 4. The manifold ofclaim 2 further comprising means receptive of electrolyte for separatelyfeeding electrolyte to said first and second headers.
 5. The manifold ofclaim 2 further comprising first distribution conduit receptive ofelectrolyte, flow bifurcating means coupled to said first distributionconduit for providing a plurality of outlets, second distributionconduit receptive of electrolyte from one of said outlets for deliveringelectrolyte to said first header, and third distribution conduitreceptive of electrolyte from another of said outlets for deliveringelectrolyte to said second header.
 6. The manifold of claim 5 whereinsaid second and third distribution conduits define physically separate,electrically isolated paths for minimizing current flow directly betweensaid first and second headers.
 7. The manifold of claim 2 furthercomprising an outer conduit having a plurality of outlets, a innerconduit concentrically disposed within said outer conduit having inletreceptive of electrolyte and outlet for discharging electrolyte intosaid outer conduit, first means for coupling said first header to one ofsaid outlets and second means for coupling said second header to anotherof said outlets.
 8. The manifold of claim 7 wherein said first andsecond means for coupling define physically separate paths.
 9. In anelectrochemical system having a a first cell and a second cell connectedelectrically in series and having a manifold for conveying electrolyteto said cells, the improvement comprising:first header for deliveringelectrolyte to said first cell and second header for deliveringelectrolyte to said second cell, wherein said first and second headersdefine physically separate, electrically isolated paths for minimizingcurrent flow directly between said first and second headers.
 10. Theimprovement of claim 9 wherein said first and second headers definepaths of substantially equal length.
 11. The improvement of claim 9further comprising means receptive of the electrolyte for separatelyfeeding electrolyte to said first and second headers.
 12. Theimprovement of claim 9 further comprising first distribution conduitreceptive of electrolyte, flow bifurcating means coupled to said firstdistribution conduit for providing a plurality of outlets, seconddistribution conduit receptive of electrolyte from one of said outletsfor delivering electrolyte to said first header, and third distributionconduit receptive of electrolyte from another of said outlets fordelivering electrolyte to said first header, and third distributionconduit receptive of electrolyte from another of said outlets fordelivering electrolyte to said second header.
 13. The improvement ofclaim 12 wherein said second and third distribution conduits definephysically separate, electrically isolated paths for minimizing currentflow directly between said first and second headers.
 14. The improvementof claim 9 further comprising an outer conduit having a plurality ofoutlets, an inner conduit concentrically disposed within said outerconduit having inlet receptive of electrolyte and outlet for dischargingelectrolyte into said outer conduit,first means for coupling said firstheader to one of said outlets and second means for coupling said headerto another of said outlets.
 15. The improvement of claim 14 wherein saidfirst and second means for coupling define physically separate paths.